Method of manufacturing liquid crystal displays having an internal polarizer

ABSTRACT

Polarizing coatings are formed from dyestuffs which provide a stable liquid crystalline phase in a wide range of concentrations, temperatures and pH-values. Particles formed by aggregates of the liquid crystal molecules are oriented in a predetermined direction to polarize light The stability of the liquid crystalline state allows orienting the particles by mechanical forces such as a shearing force applied when the liquid crystal ( 10 ) is spread on a support surface ( 20 ) by a knife-like doctor ( 90 ) or a tension deformation force acting on the meniscus of the liquid crystal deposited between two surfaces ( 20, 30 ) as the surfaces are peeled off one another. As a result, the polarizing coatings are formed in some embodiments by simple methods. In some embodiments, the polarizing coatings have a high lightfastness, a high thermal stability, and a high dichroic ratio.

The present invention relates to a method for manufacture of liquidcrystal displays according to the preamble of claim 1, and morespecifically a method for the manufacture of liquid crystal displays inwhich the polarising layers are formed internally of the cell andobtained by deposition of a liquid crystal substance.

A liquid crystal display (more commonly known by the acronym LCD) isformed by two plates of glass facing one another and separated by a gapwithin which is contained a liquid crystal substance.

On the inner surfaces of the glass plates a transparent conductive layerconstituting the electrodes is deposited, typically In₂O₃ and/or SnO₂,configured in such a way as to form the symbols which it is intended todisplay (alphanumeric characters, graphic points, icons) The resultantstructure is covered with a so-called alignment layer.

The alignment layer, generally of polyimide resin, is in contact withthe liquid crystal substance and contributes to promote an univocalorientation of its molecules over the entire contact surface.

A polarising layer is commonly disposed on the outer surfaces of theglass plates and is orientated in such a way that the directions ofpolarisation of the two layers are orthogonal to one another (in thecase of liquid crystals of the twisted nematic type) or at some otherangle (for applications of the super-twisted nematic type).

As is known in the art, back-lit or reflection displays can be made and,depending on the arrangement adopted, a further reflecting layer or aconvenient illumination device is provided on the rear plate of thedevice respectively.

The manufacturing process of a liquid crystal display comprises a firstseries of overall operations performed on a pair of glass sheets onwhich a plurality of devices (of the order of a hundred for each pair ofsheets with current technology) is defined and a second series ofoperations performed after bringing the original sheets together in afacing position and cutting or separating the cells constituting theindividual devices.

The first series of operations comprise, in succession, the steps ofwashing and sterilisation of the glass sheets, formation of theelectrode and electrical connection portions through conventionaltechniques of deposition, photolithography and etching, printing of thealignment layer and the subsequent treatment (abrasion) of its surfacein contact with the liquid crystal.

To allow the glass sheets to be joined together in such a way as to formthe gap into which the liquid crystal is introduced, while neverthelessguaranteeing the electrical connection between the plates which togetherform each display, spacer elements formed by an adhesive substance inwhich are embedded micro spheres of conductive material are convenientlydeposited by silk screen printing on one of the sheets. Then the glasssheets are joined together and the subsequent operations of cutting forseparation of the cells constituting the individual devices isperformed.

Once separation of the cells has taken place, each of these is filledwith the liquid crystal substance and subsequently sealed, then thepolarising layers are applied to the outer faces of the device incontact with the glass plates.

At the end of the series of operations described above, and before thefinal packaging, a covering and sealing operation is again performed toensure a protective anti-corrosion cladding.

Conventionally, the polarising layers consist of an adhesive film ofcellulose triacetate and are applied by simple gluing on both the outerfaces of the plates, increasing the overall thickness of the device byabout 2×200 microns (a value including the thickness of a polarisingfilm and the adhesive layer).

Recently a liquid crystal substance such as, for example, the productLCP (Liquid Crystal Polariser) commercially available by Optiva Inc. ofSan Francisco, Calif., United States, under the name TCF (Thil CrystalFilm), has been used to form polarising layers. Sub a polarisingsubstance is described in International Patent Application WO 94/28073in the name Russian Technology Group. It is a liotropic, water soluble,liquid crystal composition which has structural characteristics similarto those of the discotic lied crystal substances having molecularagglomerates of thin and elongate form. By depositing a quantity ofsubstance in liquid form on the surface on which it is desired to formthe, polarising layer, and subjecting it to stretching or otherdirectional mechanical action, its molecular structure is induced toorientate itself parallel to the axis of working so that the molecularagglomerates dispose themselves in alignment with one another assumingan overall configuration similar to that of a common polarising film.Once stretched the substance crystallises in a thin film followingevaporation of the water contained in it.

It has been determined experimentally that the polarising layers thusformed have better operative characteristics of contrast, angle of viewand reliability. The thickness of the layer (of the order of 0.5–1.0micron) is significantly, reduced and makes it possible to obtain animproved overall transmittance of the device.

A first technique for manufacturing a device with liquid crystalpolariser involves application of the polarising layer to the outerfaces of the plate of the device and its protection by means of a fixinglacquer (acrylate).

A second manufacturing technique involves, on the other hand, theapplication of the polarising layer to the inner faces of the plates andoes not therefore require any additional protective layer, rather onlya phase of “stabilisation” to render the substance insoluble in water.

For the purpose of permitting an efficient production on an industrialscale, the improvements to the production cycle are directed at makingpossible the application of the polarising layer directly onto theoriginal glass sheets, in a single general operation, before printing ofthe alignment layer and separation of the cells.

One disadvantage arises in the cutting operation for separation of theconstituent cells of the individual devices since the presence of thepolarising layer in the cut sections causes local tensions in thepolariser structure sing an imperfect cut, for example with theformation of cracking or superficial fractures.

A further disadvantage is given by the difficult and unstableapplication on the polariser of the gluing epoxy substance which isneeded for joining the glass sheets together before separation of thecells.

These disadvantages are currently resolved by exploiting the property ofthe substance used of being soluble in water. The polarising layer,after its uniform deposition, is therefore selectively removed bydelivering micro-drops of water in the regions of separation between thedisplay areas of the devices to be formed and subsequently sucking awaythe dissolved polarizing substance.

The conventional process involves selective removal by scanning theentire glass sheet with a probe including the delivery and suckingmembers, mounted on a cartesian-coordinate robot machine. The speed ofadvance is very slow (it travels at about 20–25 mm per second) and longworking times are therefore required for sheets carrying a hundreddevices (such sheets typically have dimensions of 400×400 mm). For massproduction different machines would be necessary, increasing the costsof the manufacturing process.

Another disadvantage is due to the fact that a variation in theconfiguration of the product (for example in the definition of thedisplay areas of the devices on the sheets) involves a replacement ofthe probes and the elapse of a subsequent setting up time formemorisation in the machine of new displacement steps along the axes.

Moreover, it is always necessary manually to remove excesses of materialin the perimetral region of the sheet where the cartesian-coordinaterobot machine cannot operate with precision.

In any case, removal of the polarising substance takes place by actingon the layer before stabilisation, that is to say while it still hascharacteristics of water solubility, with evident risks of damagethereof in the parts which it would be desirable to preserve.

The present invention sets out to provide a method for the manufactureof liquid crystal displays in such a way as to overcome the previouslyexplained problems.

In particular, the object of the invention is to provide a method ofmanufacture for mass production able to allow the application of alayer, of liquid crystal polariser and its rapid, precise and efficientselective removal from the separation regions between the display areasof the devices.

According to the present invention this object is achieved by a methodof manufacturing liquid crystal displays having the characteristics setout in claim 1.

The method of the invention is characterised by the fact that itexploits the conventional application of the alignment layer ofpolyimide resin, which takes place in dependence on the specificconfiguration of the areas to be covered, as a protective mask(“resist”) over the polarising layer. The polarising layer is thenselectively removed in the exposed regions, not masked by the alignmentlayer. This operation is consistent with working in a clean room in theproduction area in a which the product being produced is located andmakes it possible to exploit the apparatus normally in use in such area(immersion vessels, washing stations, drying ovens). The same printingoperation of the alignment layer is not additional, but rather normallyrequired by the process, and therefore the productive flow is notfurther weighed down or slowed and new machines are not required.

In the method according to the invention the polarising layer isstabilised (that is to say rendered water insoluble) immediately afterits application, and its removal takes place by immersion in a basicsolution and not simply by washing. Advantageously, this solution makesit possible to obtain clear-cut separation regions between the areas ofthe devices, which are free from solid residues of polarising substance,thus avoiding the formation of spurious particles within the interior ofthe finished product.

The selective removal of the layer of exposed polarising material byimmersion of the sheet in an “etching” bath also allows removal of theprint residues present on the sheet.

The entire process is conducted in the production line and makes itpossible to keep down the time taken to pass through the productionline, being demonstrably rather functional for mass production.

Further characteristics and advantages of the invention will beexplained in more detail in the following detailed description of anembodiment given purely by way of non-limitative example, with referenceto the attached drawings in which FIG. from 1 to 6 schematicallyrepresent the succession of steps of the application and selectiveremoval of the polarising layer on a sheet on which a plurality ofdisplays is defined.

In the drawings there is shown an original glass sheet 10 from which areformed the plates intended for the production of a plurality of liquidcrystal displays.

Definition of the areas of the plates and the display areas of theindividual devices, as well as the formation of electrodes andconductive paths are operations widely known in the art and will not bedescribed here, also because they are per se not relevant for thepurposes and understanding of the invention.

After formation of the electrodes, a layer 20 of polarising material inliquid form is applied over the entire sheet 10, defining theorientation of the molecules thereof along a pre-determined direction todetermine the axis of polarisation (transmission axis of the light).

The polarising layer 20 is shown in FIG. 1. Its application preferablytakes place by spreading (or similar mechanical stretching action),along the predetermined polarisation direction, of a quantity ofsubstance previously delivered in rough form. The spreading is performedby the use of a sort of doctor blade or similar distribution tool, theblade of which, having a sufficient length to extend across the entiresheet 10 in its sliding motion with respect thereto, has a corrugatedcontact profile. In the preferred embodiment the spreading tool isconstituted by a cylindrical bar on which is wound a spiral of steelwire, and has a contact profile with the substance to be spread havingthe general form of a succession of contiguous arches the convexity ofwhich faces the sheet.

Deposition of the layer with the most precise deposition techniquesmakes it possible to obtain a thickness of the crystallised polariser ofthe order of 0.5–1 micron.

Subsequently, to render the constituted polariser layer water insolublethe sheet 10 is immersed (see FIG. 2) in an aqueous solution of BaCl₂,and then washed in de-ionised water. Preferably the solution is asolution of BaCl₂ at 10% in de-ionised H₂O. The is immersion phase isfollowed by a drying phase in a static oven at 90° C. for about 20minutes (FIG. 3) to eliminate any residual moisture.

At the end of the stabilisation operations of the polarising layer 20 analignment layer of polyimide resin is applied to it by printing by meansof a mark defining a matrix of display areas 30 of the devices (FIG. 4).The alignment layer with its configuration has the additional functionof protective masking or “resist” for the polarising layer.

Thanks to the masking, the areas of the sheet 10 corresponding to thedisplay areas 30 of each device where the polariser must be maintainedare covered with extreme precision, leaving exposed the separationregions between contiguous display areas.

The polarising layer 20 is selectively removed in the exposed regionsnot masked by the alignment layer by immersion of the sheet 10 in anaqueous solution of NaOH (FIG. 5) and washing in de-ionised water in twosuccessive rousing baths. Preferably the solution is a solution of BaOHat 0.1% in de-ionised H₂O, and the immersion lasts for about 20 seconds.

The immersion phase is followed by a drying phase in a static oven at90° C. for about 20 minutes (FIG. 6) to eliminate any residual moisture.

All the operations described above are performed along the productionline in clean rooms avoiding any contamination of the sheets in thepolarising layers application phase.

At the end of these operations the conventional operation of treatmentby abrasion of the alignment layer, joining of the sheets in facingposition and cutting to separate the cells constituting the individualdevices as already described in the introductory part of thisdescription, are performed. Then, on each cell, are repeated theoperations of introduction of the liquid crystal, assembly of therespective control circuits, and final packaging of the product,according to known criteria.

Naturally, the principle of the invention remaining the same, theembodiments and details of construction can be widely varied from whathas been described and illustrated purely by way of non-limitativeexample, without by this departing from the scope of protection of thepresent invention as defined in the attached claims.

1. A method for the manufacture of liquid crystal displays, eachcomprising a pair of facing plates coupled in such a way as to form acell able to receive the liquid crystal; and a respective pair ofpolarizing layers associated with the plates, in which a plurality ofplates is formed starting from an original sheet (10), the method beingcharacterised in that it comprises the operation of application of apolarising layer (20) by deposition of a liquid crystal polarisingsubstance and orientation of the molecules of the substance fordetermination of the polarisation axis; application of an alignmentlayer of polyimide resin onto the crystallised polariser layer (20)according to a predetermined configuration substantially correspondingto the display areas (30) of the devices; and subsequent selectiveremoval of exposed portions of the polarising layer (20) from theseparation regions between the display areas (30) of the devices, notmasked by the alignment layer, the said operations being performedbefore the operations of coupling a pair of sheets (10) in facingposition and subsequent separation of the individual cells.
 2. A methodaccording to claim 1, in which the selective removal of exposed portionsof the polarising layer (20) takes place by immersion of the sheet (10)in an aqueous solution of NaOH and at least one subsequent washing.
 3. Amethod according to claim 2, in which the said aqueous solution of NaOHis 0.1% solution of NaOH in de-ionised H₂O.
 4. A method according toclaim 2, in which the immersion phase lasts about 20 seconds.
 5. Amethod according to claim 2, in which the immersion and washing phasesare followed by a drying phase at 90° C. for about 20 minutes.
 6. Amethod according to claim 1, characterised in that the polarising liquidcrystal substance is water soluble, and in that the operation forapplication of the alignment layer is preceded by a stabilisationoperation of the crystallised substance constituting the polarisinglayer (20) to render the said substance non water insoluble, the saidstabilisation operation comprising a phase of immersion of the sheet(10) in an aqueous solution of BaCl₂ and subsequent washing.
 7. A methodaccording to claim 6, in which the said aqueous solution of BaCl₂ is a10%; solution of BaCl₂ in de-ionised H₂O.
 8. A method according to claim6, in which the said stabilisation operation further comprises a dryingphase at 90° C. for about 20 minutes after the immersion and washingphases.
 9. A method according to claim 1, characterised in that theoperation for application of the alignment layer is performed byprinting by means of a mask, for definition of the display areas (30) ofthe devices.
 10. A method according to claim 1, characterised in thatoperations of deposition and orientation of the molecules of thepolarising liquid crystal substance constituting the polarising layer(20) are effected together by spreading along the polarisation directiona quantity of substance previously delivered in rough form, thespreading being achieved by means of a tool in the form of a bar havinga corrugated profile in longitudinal section, orthogonal to thedirection of spreading.